Variable filtering and timing method and system



March 12, 1946.

R. L. HAYNES VARIABLE FILTERING AND TIMING METHOD AND SYSTEM Filed Sept. i-, 1945 T W. Y T m Ir V M R L m R E N w *3 E v 6 m W u W W x E W N\ 7 m N A E %N RN f N 000 w r wm m A- w a \N Qx N\ D WI 5. A 1 3%: 4

ATTORN EY.

Patented Mar. 12, 1946 VARIABLE FILTERING AND TIIHING METHOD AND SYSTEM Robert L. Haynes, Indianapolis, Ind., assignor to Radio Corporation of America, a corporation of Delaware Application September 1, 1943, Serial No. 500,809 14 Claims. (01. 179-1003) This invention relates to electrical current transmission systems and particularly to an electronic control circuit providing variable filtering and timing characteristics for electrical transmission systems in which a current corresponding to the amplitude of a signal is utilized for control:

:plitude diminishes rapidly or ceases suddenly. In

other words, the discharge rate can be automatically controlled in such a way as to vary the filterling the transmission characteristics such as the I gain of the system, or for controlling auxiliary equipment operating in conjunction with the transmission system.

In the recording of sound for motion pictures,

the sound currents are transmitted from a pickup device, such as a microphone, to a recorder wherein a light beam is normally varied in accordance with the instantaneous values of the sound waves.

- It is well-known that in such systems a portion of the sound currents are rectified and filtered and then impressed on a noise reduction device such as light intercepting shutters or on the instantaneous light modulator itself. It is also wellknown that in various transmission circuits. such as those used for sound recording, radio broadcasts, and public address systems, that devices known as compressors and expanders are employed to vary the ratio or range of amplitudes of the si nal before recording or reproduction.

In all the above-mentioned systems, some form of rectifier is employed for obtaining a unidirectional current which varies with the amplitude of the si nal. Also, the use of a rectifier generally necessitates the use of a filter between the rectifler and its load regardless of whether the' load is a mechanical device. such as a shutter in a noise reduction unit. or a resistance for varying the grid bias of an amplifier to control the gain thereof.

Such filters include condensers and resistances and, as is well understood. the better the filtering obtained. the slower the timing and vice versa. Usually, therefore. it is necessary to reach a compromise between the optimum amount of filtering and the optimum timing characteristic. since when the timing is too slow. distortion is introduced in the form of "peak clipping" of the sound waves, and when a small amount of filtering is used to obtain fast timing, distortion is introduced in the form of flutter or spurious frequencies from the I rectifier.

The present invention is directed to an elecing characteristic according to the nature of the signal. Another feature of the invention is that no tube need be operated with its cathode floating on a high impedance circuit which makes the system reliable because leakage currents between the heater and cathode are prevented.

The principal object of the present invention.

therefore, is to improve the filtering of rectified alternating currents for control purposes.

Another .object of the invention is to improve the rectification and filtering of alternating currents used for control purposes.

A further object of the invention is to automatically vary the discharge rate of a filter condenser in accordance with the nature of a signal current.

Another object of the invention is to'prcvide an automatic control from. the output of a rectifier for varying the timing and filter characteristic of the rectifier filter.

Another object of the invention is to vary the filtering of all rectified currents in accordance with til: variation in amplitude of the signal being rect' ed.

Although the novel features which are believed to be characteristic of this invention will be pointed out with particularity in the appended claims, the manner'oi its organization and the mode of its operation will be better understood by referring to the following description read in contronic impedance control which varies in accord- 1 ance with the character of the signal; that is,

the discharge rate of the filter condenser is made slow providing good filtering during crescendos and steady tones, at which periods fast timing is neither necessary nor desirable, while a fast discharge is obtained at times when the signal ama part hereof, in which:

Fig. 1 is a schematic circuit diagram illustrat ing the basic principles of the invention.

Fig. 2 is a schematic circuit diagram illustrating a complete control circuit embodying the invention, and,

Fi 3 is a schematic circuit diagram showing a modification of the portion of the circuit of Fig. 2 to the right of the dotted line.

Referring now to Fig. 1, the signal to be rectified, which may be a signal being recorded or one being transmitted by a public address or radio broadcast system; is impressed upon the primar of transformer 5 connected to terminals 6. The signal is rectified in a rectifier 8 connected in series with the secondary of the transformer -5. The rectified impulses are filtered by condensers l0 and II and resistance l2, condenser Ill being the principal timing control.

while condenser ll andreaistance I! are the usual type of filter elements whose effect is neg ligible upon the timing. In the usual type or rectifier 2| and a pair of controlled impedance vacuum tubes 22 and 23, the impedance of which may be varied by varying the potential connected across terminals 25. When no zero bias is applied across terminals 25, the plate voltage of these tubes falls practically to zero due to the low impedance thereof, the voltage of the source being expended across resistors i6 and M. This makes point X at substantially ground potential. If only one controlled impedance tube is used such as tube 23 alone, the potential of point X might not'be reducedto a low enough value to give satisfactory operation on low amplitude signals. By using two tubes and the additional resistor H, the point X is made to approach much more closely to zero. Thus, with no bias on tubes-22 and 23, the effective resistance in shunt to condenser I ii is that provided by ref sisftiirs' w'and l 9 in series, the values of these resistors, forpurposes of illustration, being 9 with the nature of the signal.

erence is now made to Fig. 2 wherein the timing and filtering characteristics are automatically modified or varied by the signal in accordance An automatic control circuit is employed in which the quality of the filtering controls the timing. It will be noted that the portions of Figs. 1 and 2 to the right of the dotted line in each figure are idendiminishes rapidly or ceases suddenly, at which and 1 megohms, respectively. No current will now through rectifier 2i because itsplate volt 'age 'is negative. Since the effective resistance ,across condenser i0 is 10 rnegohms,'the rate of. discharge, therefore, will be slow and good filtering obtained.

To obtain a fast discharge of condenser i0,

the tubes 22 and 23 are operated at cut-off bias which increases the impedance of the tubes and I their plate potential rises to substantially 180 charge of the condenser I ll as compared with the discharge rate when both resistors i8 and l9 are in shunt thereto. At intermediate values of signal voltage, the current divides between tube 2| and resistor I8 and regardless of the signal level, providedit does not exceed 20 volts,

point Y remains at ground potential, and the efiective resistance across condenser Ill is l megohm.

Thus, the circuit'of Fig. 1 may be operated either with normal timing, which is obtained when the one megohm resistor I9 is in shunt to condenser ill, or with slow discharge timing, which is obtained when the 10 megohms of resistor l8 and I 9 in series are in shunt to condenserJll. It will be noted that in this circuit all cathodes of the vacuum tubes are grounded, thus eliminating an floating cathodes to make thesystem' unstable due to leakage currents between the heaters and cathodes of the respective 3 tubes.

Since it has been shown. that the discharge rate of the condenser 10 may be varied by changing the potential across terminals 25, reftical, except for the conductor 21 connecting the point Z through condenser 28 to the input of an amplifier 29. Also, the voltage across terminals I4 is now made 250 volts instead of 180 volts as in Fig. 1. In the systems ofFigs. 1 and 2, the output voltage of the rectifier 8 contains a certain amount of ripple, the amount of ripple depending, at any instant, upon the-nature of the signal. Now,it has been found, in operating noise reduction equipment, that, at times when the audio system is such as to produce a large amount of ripple (continued low frequency tones, and crescendos) fast timing is neither necessary nor desirable, and a real improvement infiltering is obtained by using a'slow rate of discharge at these times. Fast timing is required only at times when the signal amplitude timing, condenser 36 and resistance 31 having a large time constant, which causes a slow timing condition to' be maintained continuously, even when the ripple signal consists of short frequent pulses instead of a stead signal.

When no signal is present at point Z, tube 34 is biased to cut-off by a battery 40 or other suitable source of potential and fast timing results. As explained in connection with Fig. 1, when cut-off bias is placed on tube 34, the tube has a high impedance and only resistor I9 is effectively in shunt to condenser Hi. When ripple is present, the output of the amplifier 33, flowing through resistance 4!, shifts the grid voltage of tube 34 positive so that tube 34 operates without bias, and slow timing results, since both resistors l8 and I9 are now in shunt to condenser It].

In the operation of this circuit with zero bias,

the plate-cathode voltage drop across the tube 34 line is shown, this modification avoids a possible disadvantage of the circuit of Fig. 2. Under certain circumstances, the automatic control of illtering in the circuit of Fig. 2 may not be as eflective as desired due to the fact that when the rippi signal is obtained as shown in Fig. 2, the improved filtering resulting from producing slow discharge'timing may reduce the ripple to such an culty is avoided with the modification shown in Fig. 3, by providing a separate rectifier and filter circuit which is not controlled'by the feedback system. Therefore, in addition to the rectifier tube 8 connected to the secondary of transformer 5, a second rectifier 44 is also connected to transformer along with its timing circuit of condenser 45 and resistor 46, this rectifier, condenser, and resistor not benefiting from the improved filtering obtained in the main amplifier circuit. This auxiliary circuit has fast timing at all times, thereby providing ample ripple signal for the control circuit.

Thus, the above system will provide from any alternating current signal varying in amplitude impressed across terminals 6, a high quality unidirectional control current corresponding to the variations in amplitude of the signal for controlling noise reduction elements or the gain of a compressor or expander. High quality is obtained by automatically securing slow timing during crescendos and low frequency tones to increase the filtering above-normal when fast timing is not required and to change to fast timing when nec-' essary such as upon rapid decreasing. signals. The timing may be under direct control of the ripple in the output of a rectifier not affected by the improved results obtained to avoid degeneration. There is, thus, provided a system for automatically varying the filter and timing characteristic of a rectifier. system which may be employed in any type of electrical current transmission system where such a high quality control is desired.

I claim as my invention:

1. In an electrical current transmission system, the combination of an alternating current signal source, means for rectifying said alternating current signal to obtain a unidirectional current whose amplitude corresponds to the variations in the peak value of the amplitude of said alternating current signaL'and means for filtering said rectified current, said filtering means including impedance means and means for utilizing the ripple components of said unidirectional current for varying said impedance means. the timing, and the degree of filtering of said filter, said filtering means further including a condenser, a variable impedance in shunt with said condenser, and electronic means controlled by the amplitude of said ripple components for varying the amount of said impedance in shunt with said condenser.

2. In an electrical current transmission system, the combination of an alternating current signal source, means for rectifying said alternating current signal to obtain a unidirectional current whose amplitude corresponds to the variations in the peak value of the amplitude of said alternating current signal, and means for filtering said rectified current, said filtering means including impedance means and means for utilizing the ripple components of said unidirectional current for varying said impedance means, the timing, and the degree of filtering of said filter, said filtering means further including a condenser, a variable impedance in shunt with said condenser, and an electronic device on which said ripple components are impressed for changing said shunt impedance value and the discharge rate of said condenser to correspondingly change the timing'of said filtering means.

3. In combination, a source of alternating current signal, a rectifier for rectifying said signal, a filter connected to the output of said rectifier, said-filter including at least one timing condenser and a variable shunting resistor, and means associated with said resistor for automatically varying the resistance thereof in accordance with the amplitude of the ripple components inthe rectified current.

4. A combination in accordance with claim 3 in which said last mentioned means includes a filter connected to the output of said rectifier,

said filter including at least one timing condenser, and means associated with said condenser for automatically varying the discharge rate thereof in accordance with the nature of thesignal being rectified, said last mentioned means including a second rectifier for said signal currents, a third rectifier for the ripple components from said second rectifier, and a variable impedance device, the output of said third rectifier being connected to the input of said impedance device for varying the rate of discharge of said timing condenser in accordance with the amplitude of the ripple components from said second rectifier.

6. A system for obtaining a unidirectional current corresponding to the peak value of the amplitude of an alternating current signal comprising means for rectifying said signal currents, a condenser and a variable impedance in shunt thereto for filtering said rectified current, said rectified current containing ripple components having an amplitude depending upon the nature of said signal and the efiectiveness of said condenser and impedance filter, and means for utilizing said ripple components for varying the impedance in shunt to said condenser.

'7. A system in accordance with claim 6 in which said variable impedance includes an electronic device in shunt to said condenser, and said last-mentioned means includes an automatic control circuit for rectifying said ripple components contained in said rectifiedcurrent, said control circuit varying the impedance of said electronic device in accordance with the amplitude of the ripple present in said rectified current.

8. A system for obtaininga unidirectional current corresponding to the peak value of the am-'- plitude of a signal current comprising a first rectifying means for said signal current, means for filtering the output of said rectifying means, a second rectifying means for said signal current, a second filtering means for the output of said second rectifying means, a variable'impedance.

device connected in shunt to said first mentioned filtering means, and a third rectifying means for rectifying the ripple components present in the output of said second mentioned rectifying means, and means for connecting the output of said third rectifying means with said impedance device for varying the degree of filtering provided by said first mentioned filtering means.

9. A system in accordance with claim 8 in which said first mentioned filtering means includes a timing condenser, the discharge rate of which. is varied by the output of said third rec'- tifying means.

10. The method of automatically varying the timing and filtering characteristic of a rectifierfilter combinationfor an alternating current sig-' .nal comprising rectifying said signal, filtering said rectified signal through a condenser-variable impedance combination to a predetermined degree when said signal has a certain amplitude characteristic, and utilizing the ripple component in said rectified signal for varying the impedance value of said combination to vary the degree of filtering of said rectified signal in accordance with the variations in the amplitude characteristic of said signal.

11. The method of'obtaining a unidirectional" current corresponding to the peak value of the' parture of said signal current from said certain amplitude characteristic.

12. The method of obtaining a unidirectional control current from an alternating current signal comprising rectifying said signal, filtering said rectified currents in an amount dependent upon the amplitude variation of said signal being rectified, rectifying the ripple components of said rectifiedcurrents, filtering said rectified ripple components to a high degree, and utilizing said rectified and filtered ripple components for changing the degree of filtering of said rectified signal.

13. The method of obtaining a uni-directional control current from an alternating current signal comprising rectifying a portion of said signal to provide a first control current, filtering said first control current, rectifying another portionof said signal for obtaining a second control current, and further rectifying the ripple components of said second control current for obtaining 2. current for varying the amount of ripple components in said first control current.

14. The method of obtaining a uni-directional control current from an alternating current sig nal comprising rectifying a portion of said signal to provide a first control current, said control ond control current having a certain amount of ripple components therein, and controlling the amplitude of the ripple components in said first control current by the ripple components in said ROBERT L. HAYNES. 

